Since the 1950's, cancer diagnosis and treatment have made significant progress, particularly in the areas of identification of tumor-specific oncogenes and tumor suppressor genes.
Forty years ago, Lewis Thomas and Macfarlane Burnet proposed an immune surveillance mechanism against malignant cells. Recent studies on natural killer cells and T cells have not only provided new evidences supporting such hypothesis, but also uncovered a potential molecular mechanism underlining the immune surveillance process.
Natural killer cells (NK cells) are critical players involved in the first line of defense against pathogens and other detrimental signals. NK cells are capable of recognizing target cells and subsequently eliminating these cells through secretion of cytotoxic mediators. Since NK cells' function does not depend upon antigen/mitogen stimulation, nor does it require mediation through antibody or complement, these cells therefore should possess a recognition system to distinguish between normal and unhealthy targets. The current view is the NK cell function is regulated through a balance between its surface activating and inhibitory receptors. Major histocompatibility complex (MHC) class-1 molecules on the surface of all cells are recognized by NK cell receptors, including murine Ly49 (recognizing H-2K and H-2D) and human killer inhibitory receptors (KIR) (recognizing HLA-A, -B, -C), resulting in inhibiting NK cell's function. In viral-infected or tumor cells, these MHC class-1 molecules are frequently down regulated. A reduction of the engagement of the inhibitory receptors on NK cells causes the activation of NK cells, thereby killing these abnormal cells.
In addition to NK cells, T cells are also involved in preventing skin cancer formation induced by certain carcinogens. In human and mice, γδ-T cells in skin and gut epithelium are known to participate in local immunity. It has been shown that these T cells are involved in immune surveillance against transformation of gut epithelial cells. Moreover, recent studies have also demonstrated that these T cells are important players in eliminating transformed cells induced by exogenous carcinogen. The induction of two MHC class-I related molecules, MIC-A and MIC-B, on abnormal cells has been shown to be involved in the immune surveillance processes.
Bauer et al (Bauer S, et al., Science 1999 Jul 30; 285(5428): 727-9) have identified NKG2D as a receptor for MIC-A and MIC-B through representational differential analysis (RDA), which is a orphan C-type lectin-like NK cell receptor with unknown expression and function.
Several NK cell receptors, which are specific to MHC-1 or the MHC-1-related molecules, have been found. Unlike other NK cell receptors, NKG2D is an activating receptor present on all NK cells, γδ-T cells, and some CD8+ T cells. It forms a receptor complex with a transmembrane signaling adaptor, DAP10, in which its cytoplasmic domain contains a YxxM sequence motif capable of activating PI3 kinase-mediated signaling pathways (Wu J, et al., Science 1999 Jul. 30; 285 (5428): 730-2).
There is no MICA or MICB homologue in mice. However, it was subsequently found in mice that a family of glycoproteins called RAE-1 also served as ligands for murine NKG2D (Cerwenka A, et al., Immunity 2000 June; 12 (6): 721-7). Recent studies have revealed that there are at least five RAE-1 molecules (RAE-1-α, -β, -γ, -δ, -ε) and one RAE-1-related molecule H60. These molecules are absent or low expressed in normal tissues but are highly expressed in certain malignant tissues or upon treatment with retinoic acid. Their expression can also be found in tumors induced by carcinogen TPA. Recent studies by Diefenbach et al. (Nature 2001 Sep. 13; 413(6852): 165-71) and Cerwenka et al. (Proc Natl Acad Sci USA 2001 Sep. 25; 98 (20): 11521-6) have demonstrated that transfection of RAE-1 in MHC class-I expressing tumor cells results in rejection of these tumors by NK cells in mice. Similar to NK cells, murine γδ-T cells can also kill inoculated squamous carcinoma cell line in vivo via NKG2D, and under certain experimental conditions, RAE-1 can induce γδ-T cell memory response against transplanted tumors.
The subsequent studies have uncovered RAE-1 homologues in human, including ULBP-1, ULBP-2, and ULBP-3. These molecules were initially identified as interacting partners with human cytomegaloviral glycoprotein UL16. Although ULBP molecules are related to MHC class-1, they are not close to MICB, which is also capable to bind UL16. ULBP is also the ligand for NKG2D and is capable to stimulate NK cell to express cytokine and chemokine. The expression of ULBP in target cells against NK cells prevent them from attacking by NK cells. In the cytomegalovirus infection, the ULBP or MIC antigen may be veiled by UL16 protein so as to avoid the attacks from the immune system.
Taken together, numerous studies in human and mice indicate NKG2D plays a vital role in immune responses mediated by NK cells, γδ-TCR+T cells, CD8+αβ-TCR+T cells against virus and tumors.
Since the 1990's, research in tumor immunology has made some breakthroughs. Immune treatment of cancer comes to a new era. Many immune treatments have been in clinical level, mainly by activating the immune cells of patients in vivo or in vitro to recognize malignant cells. The further proliferation of such immune cells eliminates or inhibits the growth of malignant cells. In 1991, human tumor rejection antigen was discovered for the first time. From then on, numerous studies indicate that most of the tumor cells have different molecules from normal cells, which could be recognized and attacked by immune system. Such molecules are called tumor rejection antigen. Human immune cells capable of tumor killing can be induced in vivo and in vitro by tumor rejection antigen. Therefore, tumor rejection antigens are the most important component in tumor immune treatment.
So far, several tumor antigens have been discovered in melanoma and other tumor tissues including prostate tumor, thymus tumor, ovarian tumor and gastrointestinal tumor. Tumor rejection antigens discovered by now are classified into four types. The first type is from somatic mutations of normal genes and the second is from the mutation of genes related to tumor progressing. These two types have patient specificity, which are not available for generalized treatment. The third type of antigen expresses in normal tissues but their expression levels are highly elevated in tumors. If their genes are not mutated, these antigens are universal in tumor patients. However, they have strict tissue specificity rather than tumor specificity so that they do not have significance in clinical treatment. The fourth type has strict tumor specificity and is related to tumor progression. Because they are widely expressed in humor tumor, these tumor antigens are suitable candidates to be tumor markers and targets of anti-tumor immune response. However, very few antigens discovered by now belong to the fourth type.
Therefore, there is a keen need in the art to develop new tumor rejection antigen of the fourth type, which can be used as a tumor tag for tumor diagnosis and treatment.